Plasticity of NMDA receptor NR2B subunit in memory and chronic pain

Abstract

Glutamatergic synapses play critical roles in brain functions and diseases. Long-term potentiation (LTP) is a most effective cellular model for investigating the synaptic changes that underlie learning as well as brain disease--although different molecular mechanisms are likely involved in LTP in physiological and pathological conditions. In the case of learning, N-methyl-D-aspartate (NMDA) receptor is known to be important for triggering learning-related plasticity; alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) receptors are thought to be important for the expression of synaptic changes. In this review, I will examine recent evidence on the novel roles of NMDA receptors, in particular NR2B subunit-containing NMDA receptors in learning and chronic pain. A positive feedback control of NR2B receptor subunit is proposed to explain cortical sensitization involved in chronic pain, but not learning and memory.

Figure 1

Genetic overexpression of NMDA NR2B receptors in forebrains enhanced central synaptic potentiation. a. Synaptic potentiation induced by a single tetanic stimulation (100 Hz, 1 sec) was significantly enhanced in NR2B transgenic mice compared with wild-type littermates; b. Enhancement of synaptic potentiation is frequency-dependent, and LTD induced by low-frequency stimulation (1 Hz, 15 min) was not affected din NR2B transgenic mice. Modified from Tang et al (1999). Zhuo's lab contributed to the plasticity studies in this original article.

Figure 2

Selective enhancement of chronic pain in 'smart' mice with NR2B overexpression in forebrains. a. Overexpression of NR2B mRNA in pain-related forebrain areas including the ACC and insular cortex (IC). No overexpression of NR2B was detected in the spinal cord. b. Enhancement of behavioral nociceptive licking responses to peripheral subcutaneous injection of formalin in NR2B transgenic mice. c. Summarized three different phases of behavioral nociceptive licking responses in wild-type and NR2B transgenic mice. Similar increases in behavioral nociceptive responses were found in the second line of NR2B transgenic mice. Modified from Wei et al. (2001).

Figure 3

Peripheral inflammation triggers long-term increases in NMDA NR2B receptor mediated currents in cingulate pyramidal cells. NR2B sensitive component of NMDA receptor mediated EPSCs were enhanced in mice with CFA injection. a. Traces of EPSCs show the currents at different time points during application of drugs. Ro 25–6981 produced its maximal effect at 3 min after bath application, and a higher dose of Ro 25–6981 (3 μM) had no additional effects. The remaining currents can be totally blocked by AP-5 (50 μM). b. A selective NR2B antagonist, Ro 25–6981, partially inhibited NMDA receptor-mediated EPSCs. The time course of changes in EPSC amplitude before and during the application of Ro 25–6981 (0.3 and 3 μM) and AP-5 (50 μM) in ACC neurons from both saline (control, open symbols) and CFA-injected (filled sumbols) mice is shown. Modified from Wu et al. (2005).

Figure 4

Central signaling pathways contribute to chronic pain. In the ACC, glutamate is the major fast excitatory transmitter between input fibers and pyramidal cells. Peripheral injury such as tissue inflammation or nerve injury trigger a burst of abnormal activity in the ACC circuits; and subsequently activate postsynaptic NMDA receptors on cingulate pyramidal cells located in layer II-III. Activation of NMDA receptor triggers calcium influx. In adult ACC pyramidal cells, most of NMDA receptors are the combination of NR1-NR2A, NR1-NR2B with possible minor component of currents made of NR1-NR2A-NR2B. Postsynaptic increases in Ca²⁺ leads to activation of Ca²⁺-calmodulin (CaM) dependent pathways. Among them, Ca²⁺ and CaM stimulated AC1 is activated, and this activation leads to the generation of the key second messenger cAMP. Subsequently, cAMP activates PKA. PKA then translocates to the nucleus and phosphorylates CREB. NR2B contains a CREB binding domain which may couple increases in intracellular calcium with the increase in NR2B expression. Subsequently, postsynaptic synthesis of NMDA NR2B is increased, and together with endogenous motor protein KIF17, these new NR2B subunits are added to postsynaptic NMDA receptors. Such positive feedback control may further enhance neuronal excitability within the ACC, and contribute to chronic pain.